"Alien" parasite in tail

I've asked a few folks about this one, and no one has an answer. Hopefully someone here can.

May 31st, we put our 47 koi back outside into their pond. (It's cold here in the winter!) We have opportunity to inspect each one, and except for minor bumps all were in fine health.

3 days later, one fish, an 8-yr-old, presented with four small bumps on his tail.
Today those bumps have "grown" into one long bump that is about 1" long and about 1/4" thick. On one end of the bump, it penetrates through the fin into a point on the other side.
The fish does not appear bothered -- eating well, swimming well, and the initial redness that surrounded the bumps has disappeared for the most part.
Water quality is good (Ammonia 0, Nitrites 0, Nitrates 0, Salt .08%, pH 7.5 and steady, KH is within range) and although some algae is developing in the hot sun, there are no other indicators present. None of the other fish have the same thing, and all moving related bruises have healed.

I can post photos taken a week or so ago. Haven't got another good opportunity since.
Any help identifying this would be helpful. Never seen it before -- hence the "alien" description in the title!
Thanks!

We can't get a "feel" of it -- pond is too big (15,000 gal) and netting him right now is not an option. However it doesn't match the pox description. It definitely looks like something under the skin - especially the point where it goes through the fin. It's a raised bump - cylindrical in shape.

Salt is low ... however, trying not to shock new plants. Will raise it slowly over next few weeks. I have salt-tolerant plants as well. Need them as filters.

We can't get a "feel" of it -- pond is too big (15,000 gal) and netting him right now is not an option. However it doesn't match the pox description. It definitely looks like something under the skin - especially the point where it goes through the fin. It's a raised bump - cylindrical in shape.

Salt is low ... however, trying not to shock new plants. Will raise it slowly over next few weeks. I have salt-tolerant plants as well. Need them as filters.

We're monitoring him every day through out the day.

Ok, now I'm curious about the salt too. Spring start up assistant? Or do you maintain salt in your pond year round?

We maintain some salt in the water during the summer. Our fish are indoors in the winter, so their immune systems are not shut down in the Spring - which is something that fish outside year-round experience.

However, being outdoors again, they will come into contact with natural problems they don't get indoors. Salt helps them on a general basis -- all fresh water fish for that matter -- require some salt in the water. We use salt to combat parasites, but that would be at a much higher concentration -- and that kills most of the plant life that are not salt tolerant. As a piece of trivia, both Umbrella Palms and Papyrus are salt tolerant to .3%.

I've taken new hi-res photos today with a new camera. Once I figure out how to get them off the camera, I'll upload them. The "alien" has evolved!

I asked about the salt because I thought maybe you were adding it for the “alien”. Koi do not need salt year round. If you feel they need it when they first go outside, you could use it for a few weeks but it shouldn’t be used all the time.

If it was my fish I'd knock it out and see if I could remove it with tweezers, swab the area with some peroxide, antibiotic, a little denture powder and put her back in clean water. You may get someone more knowledgeable with a better answer so don't rush to do this.